Distribution of Sigma factors delineates segregation of virulent and avirulent Mycobacterium

Abstract

Sigma factors, in combination with RNA polymerase and several transcription factors play specific role in expression of housekeeping as well as various stress responsive genes in mycobacterial species. The genus Mycobacterium includes a wide range of species under major pathogens, opportunists and non-pathogens. The number and combination of sigma factors is extremely diversified among Mycobacterium. We have performed comparative genome analysis among 40 different species of Mycobacterium whose whole genome sequence is available, in order to identify the distribution of sigma factors. The study illustrate that SigC, SigD, SigG, SigH, SigK and SigI are dominant among the true pathogens. Moreover, 16S rDNA based phylogenetic analyses distinctly differentiate the slow growing Mycobacterium from the fast growers, and clusters the true pathogens from the opportunists and non-pathogens. While evaluating the similarity coefficient upon the allotment of sigma factors of different Mycobacterium species through UPGMA dendrogram analysis, it is apparent that the true pathogens are grouped separately following the similar trend observed from evolutionary approach. Sigma factors playing dominant role in pathogenicity are found stable in nature with high aliphatic index thereby remain flexible at a wide range of temperature. The comparative distribution of six well known virulence factors of Mycobacterium - PhoP, PcaA, FbpA, Mce1B, KatG and PE_PGRS and various sigma factors justify the allotment pattern of mycobacterial sigma factors among pathogenic species. The pathogenicity responsible sigma factors elicit close resemblance with few notable characters of the known virulence factors. Thus the analysis renders that the distribution of sigma factors of different species of Mycobacterium can be a potential tool to predict the pathogenicity index of this genus.